1. Field of the Invention
The present invention relates to an exhaust gas recirculating system for an internal combustion engine. In particular, the invention concerns an apparatus for controlling recirculation of the exhaust gas in dependence on pressure prevailing in an intake pipe of the internal combustion engine.
2. Description of the Prior Art
In general, the internal combustion engine for motor vehicles is provided with an exhaust gas recirculating system through which a part of exhaust gas discharged from the internal combustion engine (hereinafter also referred to simply as engine) is recirculated or fed back to the engine with a view to decreasing the content of nitrogen oxides (NO.sub.x) carried by the exhaust gas by lowering the maximum temperature of combustion taking place in the engine to thereby suppress production of nitrogen oxides.
In order to have a better understanding of the invention, a hitherto known exhaust gas recirculating system will first be described by referring to FIG. 1 of the accompanying drawings. In this figure, reference numeral 1 denotes an intake conduit or manifold leading to the intake ports of an internal combustion engine (not shown), and numeral 2 denotes an exhaust manifold or pipe extending from the discharge side of the engine. It will be noted that the exhaust pipe 2 is connected to the intake conduit 1 through an exhaust gas recirculating passage 3 in which a recirculation control valve assembly generally denoted by a numeral 4 is disposed. The recirculation control valve assembly 4 is composed of a valve 5 adapted to be closed or opened for controlling flow of exhaust gas recirculated or fed back to the engine and a diaphragm device 6 serving as an actuator for driving the valve 5 in a direction to open or close the exhaust gas recirculation passage 3. The diaphragm type valve actuator 6 includes a partition diaphragm 6A which divides the inner space of a housing of the valve actuator 6 into a lower chamber 16 and an upper chamber 7. The lower chamber 16 is communicated to the atmosphere through an opening 16A, while the upper chamber 7 is communicated to the intake passage or manifold 1 through a vacuum pressure passage or pipe 8 which is opened in the intake passage 1 at a position downstream of and in the vicinity of a throttle valve 9. More particularly, the passage 8 is opened at such a location which is positioned upstream of the throttle valve 9 in the fully closed state thereof while taking a position downstream of the throttle valve 9 when the latter is opened more or less. Thus, the upper diaphragm chamber 7 is applied with a vacuum pressure prevailing in the intake conduit 1. Reference numeral 18 denotes a bias spring which is so set that a desired rate of recirculation can be attained.
With the arrangement of the exhaust gas recirculation system described above, it will be seen that the quality of exhaust gas flow fed back to the engine is varied in dependence on the vacuum pressure prevailing in the intake passage 1 downstream of the throttle valve 9. More particularly, when the vacuum pressure in the intake passage 1 is increased due to a decreased opening degree of the throttle valve 9, the valve element 5 is displaced upwardly by the diaphragm 6A to allow the flow of recirculated gas to be correspondingly increased. In this manner, in low and intermediate load ranges of the engine where the opening degree of the throttle valve 9 is relatively small, the valve 5 is opened to a greater degree to permit a correspondingly increased recirculation of the exhaust gas.
In conjunction with the exhaust gas recirculation system described above, it should be mentioned that a fine particle trap device known as a filter trap or the like for catching or trapping fine particles entrained by the exhaust gas is installed in the exhaust pipe at a position downstream of the recirculating passage 3 in some types of the internal combustion engines such as Diesel engine, for example. In this connection, it will readily be understood that as the trapping of particles proceeds, the filter trap device is gradually clogged, and as a result the gas transmitivity of the filter trap device is progressively reduced so that resistance to the exhaust gas flow is correspondingly increased, thus giving rise to an increase in the exhaust gas pressure in the exhaust pipe upstream of the trap device. Under the circumstances, the differential pressure appearing across the valve 5 is also increased correspondingly, thus resulting in a relatively high ratio of exhaust gas recirculation. In other words, the presence of the fine particle trap device will cause the recirculation ratio to be deviated from the optimum value as time elapses. This is of course a problem or difficulty to be eliminated.